Table 1 Differential diagnosis of columnar cell change, columnar cell hyperplasia, flat epithelial atypia and atypical ductal hyperplasia.

Feature	CCC	CCH	FEA	ADH
Cell morphology	Columnar	Columnar	Columnar and/or cuboidal	Cuboidal
Number of cell layers	⩽2	>2	Variable	Variable
Architecture	Flat	Tufts/mounds	Flat or tufts/mounds. Not complex*	Complex architectural pattern*
Nuclear atypia†	Absent	Absent	Present	Present
Nuclear features†	Ovoid, bland, polarised nuclei arranged perpendicular to basement membrane	Ovoid, bland; some nuclear crowding and overlapping of nuclei may be observed. Nuclei arranged perpendicular to basement membrane	In typical form: round, uniform, evenly‐spaced with finely dispersed chromatin, as in low grade DCIS. Alternatively may show more “classical” cytonuclear atypia in the form of enlarged, more oval nuclei with mild to moderate pleomorphism; some clumping of chromatin may be seen	Round and evenly spaced as in low grade DCIS; finely dispersed chromatin
Nuclear to cytoplasmic ratio	Normal	Normal	Increased	Increased
Internal contours of acini	Irregular	Irregular	Typically smooth	Variable

ADH, atypical ductal hyperplasia; CCC, columnar cell change; CCH, columnar cell hyperplasia; FEA, flat epithelial atypia; DCIS, ductal carcinoma in situ.

*True micropapillae (with bulbous tip extending into the lumen from a narrower stalk) or sieve‐like cribriform spaces.

†Cases with high grade nuclear atypia should be classified as flat high grade DCIS and do not fall within either the spectrum of FEA or of ADH.